Method of doping organic semiconductors

ABSTRACT

A method includes the steps of forming a contiguous semiconducting region and heating the region. The semiconducting region includes polyaromatic molecules. The heating raises the semiconducting region to a temperature above room temperature. The heating is performed in the presence of a dopant gas and the absence of light to form a doped organic semiconducting region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No.12/024,484 filed on Feb. 1, 2008, to Kloc, et al. entitled “Method ofDoping Organic Semiconductors”, currently allowed, commonly assignedwith the present invention and incorporated herein by reference in itsentirety. The present application is related to U.S. patent applicationSer. No. 11/375,833 to Kloc, et al. entitled “Fabricating Apparatus withDoped Organic Semiconductors”, which is commonly assigned with thepresent application and hereby incorporated by reference as ifreproduced herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contract No.DE-FG02-04ER46118 awarded by the Department of Energy.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to organicsemiconductors.

BACKGROUND OF THE INVENTION

Organic semiconductors are the subject of intense research interest.Potential benefits of these materials include low-cost, wide areacoverage, and use with flexible electronic devices. They have beenemployed in organic light-emitting diodes (oLEDs) and organicfield-effect transistors (oFETs), and in circuits integrating multipledevices. Fabrication techniques such as ink-jet printing have helpedreduce the cost of fabrication of these devices and integrated circuitsusing them.

SUMMARY OF THE INVENTION

One embodiment is a method that includes forming a contiguoussemiconducting region that includes polyaromatic molecules. The methodfurther includes heating the region to a temperature above roomtemperature in the presence of a dopant gas and the absence of light toform a doped organic semiconducting region.

Another embodiment is a method that includes forming an organicsemiconducting region that includes a crystalline region of polyaromaticmolecules. A dielectric layer is formed over the organic semiconductingregion. The method further includes forming an opening in the dielectriclayer to expose the organic semiconducting region. The organicsemiconducting region is then heated to a temperature above roomtemperature in the absence of light.

Another embodiment is a method that includes forming a crystallineorganic semiconducting region that includes polyaromatic molecules. Asource electrode and a drain electrode are placed in contact with theorganic semiconducting region. A gate electrode is located to affect theconductivity of the organic semiconducting region between the source anddrain electrodes. The method further includes forming a dielectric layerof a first dielectric between the organic semiconducting region and thegate electrode. The dielectric layer is substantially impermeable tooxygen and in contact with the organic semiconducting region. An openingis formed in the first dielectric to form a doping channel, wherein aportion of the organic semiconducting region is in contact with a seconddielectric via the opening. A physical interface is located between thesecond dielectric and the first dielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are understood from the following detaileddescription, when read with the accompanying figures. Various featuresmay not be drawn to scale and may be arbitrarily increased or reduced insize for clarity of discussion. Reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

FIGS. 1A-1D illustrate an embodiments of a FET;

FIGS. 2 and 3 illustrate organic semiconducting molecules;

FIGS. 4A-4D and 5A-5B illustrate electrical characteristics of a FET;

FIGS. 6A-6F illustrate an embodiment of a method of forming a FET; and

FIG. 7 illustrates an embodiment of a method of doping a FET.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Some polyaromatic semiconductors have been found to have relatively poorstability of electrical properties in the presence of some gases.Oxygen, e.g., can react with a portion of the polyaromatic molecule,thereby altering the electronic properties of the molecule. Suchinstability may be regarded as undesirable in many circumstances.

Some of the embodiments recognize the unexpected benefits of increasingthe conductivity of a semiconducting polyaromatic layer by exposure to adopant gas while heating and excluding light. In some embodiments, theincrease of conductivity is substantially due to an increase of carrierconcentration without a significant change of mobility of the carriers.Some embodiments stabilize the conductivity of the layer by subsequentexclusion of light and/or further exposure to the dopant gas from thelayer using, e.g., a barrier layer or a package.

FIG. 1A illustrates a cross-sectional view of an embodiment of anorganic field effect transistor (FET) 100. The FET 100 includes achannel material 110 having a surface 115. A source electrode 120 and adrain electrode 130 are in contact with the channel material 110, and achannel region 140 is shown as a contiguous portion of the channelmaterial 110 between the source/drain electrodes 120, 130. A dielectriclayer 150 with a thickness 155 substantially encapsulates the surface115 of the channel material 110, with a portion 160 of the channelmaterial 110 exposed to the ambient by virtue of an opening in thedielectric layer 150 that forms a doping channel 165. In the illustratedembodiment, the channel material 110 is self-supporting. In suchembodiments, the FET 100 may optionally be affixed to a substrate. Insome embodiments, the doping channel 165 may be formed, e.g., bycleaving the channel material 110. The channel region 140 is a distance170 from the doping channel 165. As discussed further below, in someembodiments the dielectric layer 150 is impermeable to a dopant gas usedto dope the channel material 110. A gate electrode 180 is formed overthe dielectric layer 150 between the source/drain electrodes 120, 130.

Exposure to the dopant gas increases the majority charge carrier densityin the channel material 110 of the organic semiconductor as describedherein. It is believed by the inventors that when incorporated into theorganic semiconductor, the dopant gas operates to transfer mobilecharges to the organic semiconductor, thereby creating charge traps. Insome cases, the dopant gas molecules may form covalent bonds with theorganic semiconductor. In other cases, the dopant gas molecules mayoccupy interstitial sites in the organic semiconductor withoutcovalently bonding thereto. Depending on the gas molecule and theorganic semiconductor, the charge traps may be positive or negativeions. The exposure to the dopant gas may increase the majority chargecarrier density while leaving the mobility of the charge carrierssubstantially unchanged. As used herein with respect to the chargecarrier mobility, substantially unchanged means that the mobilitychanges less than about 10%. In some cases of substantially unchangedmobility, the mobility may decrease, e.g., by less than 5% or by lessthan 1%.

In some cases, the dopant gas comprises oxygen. In such cases, the gasmay be, e.g., a homoatomic source of oxygen, such as O₂ or O₃, or aheteroatomic source such as H₂O or N₂O. In other cases, the dopant gasmay comprise a halogen, e.g., F, Cl, I or Br. Doping by the dopant gasmay be reversible by non-chemical means, e.g., by exposing the dopedchannel material 110 to elevated temperature. Such exposure may reversedoping by, e.g., causing outgassing of the dopant gas by the dopedorganic semiconductor. In some cases, doping may be reversed by exposingthe doped organic semiconductor to a reducing gas, e.g., H₂.

The term impermeable as used herein with respect to a dielectric layeror barrier means that the rate of diffusion of the dopant gas throughthe layer barrier is below a rate that results in a significant changeof semiconducting characteristics of the channel region 140 over theoperational lifetime of a device employing the channel material 110. Insome cases, an operational lifetime of such a device is about 10 years.A significant change as used with respect to semiconductingcharacteristics is a change that causes the device, such as a FET, tooperate outside its operational specifications. Such a change may be,e.g., a 5% change of conductance at a given gate voltage V_(gs) from theconductance at a given threshold voltage immediately after manufacture.

The channel material 110 is referred to as “exposed” when the portion160 forms an interface with a second dielectric. The second dielectricmay be, e.g., the surrounding ambient or another layer of a soliddielectric material. In the latter case, the solid dielectric materialmay be the same or a different material as the dielectric layer 150.

The FET 100 may be a component of an apparatus 190. The apparatus 190may additionally include other electronic devices, such as resistors,capacitors and transistors, and a power source to operate the FET 100.The FET 100 may optionally be configured to electrically behaveprimarily as, e.g. a transistor, resistor, capacitor, or LED.

FIG. 1B illustrates an embodiment of the FET 100 in which the channelmaterial 110 is in contact with a substrate 195. The dielectric layer150 is formed over the channel material 110 and the substrate 195. Whenthe dielectric layer 150 is impermeable to the dopant gas, the substrate195 is preferably at least as impermeable to the dopant gas as is thedielectric layer 150. Substrate materials such as glass, silicon andsome polymers are sufficiently impermeable to most gases and may alsoprovide mechanical support to the channel material 110. In theillustrated embodiment, the doping channel 165 may be formed, e.g., by aplasma etch process designed to remove the dielectric layer 150. Such aprocess may optionally be designed to stop on the channel material 110,as shown. The source electrode 120 and the drain electrode 130 areformed on the channel material 110, and thus lie between the dielectriclayer 150 and the channel material 110.

FIG. 1C illustrates an embodiment in which the source electrode 120 andthe drain electrode 130 are formed on the substrate 195. The channelmaterial 110 is formed over the source/drain electrodes 120, 130 and thedielectric layer 150 is formed thereover. The doping channel 165 mayagain be formed by a plasma etch process. In this case, the etch processmay be designed to remove the dielectric layer 150 and the channelmaterial 110 to form the doping channel 165. In the illustratedembodiment, the exposed portion 160 includes a larger surface area ofthe channel material 110 than does the embodiment of FIG. 1B. Thus, theconfiguration of FIG. 10 may provide more rapid doping of the channelmaterial 110 with the dopant gas than the configuration of FIG. 1B.

FIG. 1D illustrates an embodiment in which an optional capping layer 197is formed over the dielectric layer 150 and the doping channel 165. Thecapping layer 197 may serve to prevent exposure of the channel material110 to the doping gas after doping the channel material 110 as describedbelow. The capping layer 197 may also have other characteristics thatare desirable in some circumstances. For example, the capping layer 197may be chosen to exclude light from the channel material 110 that iscapable of forming or breaking molecular bonds or causing molecularexcitations therein.

The capping layer 197 may be the same or a different material from thedielectric layer 150. The choice of material for the capping layer 197may be influenced by, e.g., the ability to fill the doping channel 165,opacity at a wavelength of interest, and barrier properties. Regardlessof whether the dielectric layer 150 and the capping layer 197 are thesame or a different material, an interface 198 is formed between thedielectric layer 150 and the capping layer 197. When the dielectriclayer 150 and the capping layer 197 are the same material, the interface198 may be detected, e.g., by electron microscopy. In some cases, thefunction of the capping layer may be provided by a package that excludeslight and the doping gas, e.g., oxygen.

The channel material 110 includes a crystalline or polycrystallineorganic semiconductor, which may be a p-type or an n-type semiconductingmaterial. For a p-type material, e.g., when a voltage V_(gs) of the gateelectrode 180 with respect to the source electrode 120 is at or below athreshold voltage, V_(th), of the FET 100, the channel region 140becomes more conductive, and current may flow between the sourceelectrode 120 and the drain electrode 130. The V_(th) depends on thedielectric permittivity of the dielectric layer 150 and the dielectricthickness 155. While not limiting the scope of the invention by theory,in the case of p-type organic semiconductors it is believed that belowV_(th), a charge trap, e.g., an electron trap state or an acceptorstate, localizes thermally activated electrons from the valence band,and remaining delocalized holes produce p-type semiconducting propertiesof the channel material 110.

The channel material 110 is either a single crystal of polyaromaticmolecules or a polycrystalline layer of the molecules. The polyaromaticmolecules can be members of two broad classes. The first of theseclasses includes monodisperse compounds incorporating a plurality ofaromatic or heteroaromatic units, where the units may be fused to eachother and/or linked to each other in a way that maintains conjugation ofπ-bonds. Conjugated π-bonds provide for delocalization of electrons inthe polyaromatic molecules. The second class includes polymers andoligomers having the aforementioned polyaromatic characteristics.Herein, oligomers are polymer chains with less than about 10 repeatingunits. The polyaromatic molecules in these classes are typicallycharacterized by having p-type semiconducting properties in the solidphase. Numerous such molecules are known in the art. For example, suchmolecules include acenes, thiophenes, di-anhydrides, di-imides,phthalocyanine salts, and derivatives of these classes of molecules.

Acenes are polyaromatic compounds having fused phenyl rings in arectilinear arrangement, e.g., three or more such fused rings. Asubclass of acenes includes those in which the aromatic rings arearranged in a linear fashion, as shown below. Among the linear acenesinvestigated for semiconducting applications are tetracene (n=2) andpentacene (n=3).

Thiophenes are molecules that have a five-member ring containingsulphur. Thiophenes having p-type semiconducting characteristics includethose having one or more fused phenyl rings arranged in a linearfashion, with a terminal fused thiophene ring. A general structuralrepresentation of thiophenes having two terminal thiophene rings isshown below, for which n=0, 1, 2 . . . .

FIG. 2 shows examples of polyaromatic molecules with semiconductingproperties that can be used as the channel material 110. These examplesinclude: pentacene 210 and processable derivatives thereof such as6,13-bis (triisopropylsilylethynyl) pentacene (TIPS) 220; processablederivatives of anthradithiophene 230 and benzodithiophene 240;5,6,11,12-tetraphenylnaphthacene (rubrene) 250 and processablederivatives thereof; naphthalene-1,4,5,8-tetracarboxyl di-anhydride 260;and derivatives 270 of N-substituted naphthalene-1,4,5,8-tetracarboxylicdi-imide.

FIG. 3 shows examples of polyaromatic oligomers and polymers that can beused as the channel material 110. The examples include:poly(9,9-dioctylfluorene-alt-bithiophene (F8T2) 310; poly(3,3′-dioctylterthiophene) (PTT8) 320; regioregularpoly(3-hexylthiophene) (P3HT) 330; poly(9,9-dioctylfluorene) (F8) 340;poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) 350; andoligomeric polyaromatic molecule oligothiophene 360, and derivativesthereof. Those skilled in the pertinent art will appreciate that theabove examples of polyaromatic molecules are not exhaustive of suchmolecules.

Additional details regarding semiconducting polymers and applications toFETs may be found in U.S. patent application Ser. No. 11/375,833 toKloc, et al., previously incorporated by reference.

Crystals and films of semiconducting organic molecules do not typicallyhave a significant population of electrons and holes in equilibrium inthe absence of an applied electric field. Hence, the conductivity ofsuch molecules is generally low relative to inorganic semiconductors.For example, while intrinsic silicon has a conductivity of about 1.5E-5Ω⁻¹cm⁻¹, intrinsic pentacene may have a conductivity of about 1.8E-8Ω⁻¹cm⁻¹ and intrinsic rubrene may have a conductivity of about 1E-9Ω⁻¹cm⁻¹.

However, exposure of some organic semiconductor films to a dopant gascan change the conductivity and mobility of charge carriers in thefilms. In many cases, these changes are detrimental to the purpose forwhich the film is used. Such exposure may be advantageous, however, ifdone in a controlled manner as described herein to produce stablesemiconducting characteristics.

Embodiments disclosed herein benefit from the recognition that anorganic semiconductor can be doped with a dopant gas in a manner thatresults in a substantially stable mobility of charge carriers.Previously known doping methods generally result in a substantialdecrease of the mobility of the charge carriers. Without limitation bytheory, it is thought that the decrease of mobility in prior art dopingmethods results from formation of deep charge traps. It is furtherthought that the embodiments described herein result in shallower chargetraps that reduce hole mobility to a lesser degree. When the mobility isconstant, or nearly so, a change of conductivity is about proportionalto a change of the carrier concentration. Thus, the ability to changecarrier concentration without substantially changing mobility,heretofore unknown, simplifies the task of designing circuits using suchFETs, and provides the designer with more predictable operationalcharacteristics of the FET.

In an embodiment, energy for the doping reaction is provided by raisingthe temperature of the organic semiconductor above room temperature(e.g., about 25° C.). The minimum reaction temperature will, in general,be related to the dopant gas and the organic semiconductor, as differentdopant gases and materials will in general have different activationenergies associated with forming charge traps therein by theirinteraction. A maximum doping temperature will in general be related tothe onset of decomposition, or nonuniform or irreversible reactionsbetween the dopant gas and the organic semiconductor. In one embodiment,in which O₂ is the dopant gas and the organic semiconductor is rubrene,the doping reaction may be performed at a temperature ranging from about90° C. to about 150° C. In some cases, the doping reaction may beperformed in a narrower temperature range of about 105° C. to about 115°C. For embodiments which use a dopant gas other than O₂, the dopingtemperature range may include temperatures above 150° C. and below 90°C.

In addition to exposing the organic semiconductor to elevatedtemperature, light or other type of electromagnetic radiation may beexcluded during such exposure. As used herein, light refers to photonswith sufficient energy to induce an excited molecular state of theorganic semiconductor molecule. The excited molecule may then react withthe dopant gas to produce, e.g., an endoperoxide or other oxygen-relateddefect when an oxygen-containing dopant gas is used.

In the context of the present discussion, the formation of theendoperoxide or other deep charge trap is considered undesirable. Thus,the method described herein excludes photons capable of producing amolecular excitation that may result in formation of such traps. Photonswith a wavelength long enough that only heating, e.g., of thesemiconductor occurs are not considered “light” in the presentdiscussion. In some cases, light only includes photons having energygreater than about 3 eV, corresponding to a wavelength of about 400 nmor less. This wavelength range corresponds roughly with ultraviolet andhigher energy photons. In other cases, light includes visible photons,e.g., having a wavelength up to about 780 nm. Such cases include thosein which visible wavelengths are capable of producing molecularexcitations in the organic semiconductor molecule capable of reactingwith the dopant gas to produce deep charge traps.

Doping as used herein includes both additive doping and subtractivedoping. Additive doping involves increasing the density of charge trapsin an organic semiconductor, while subtractive doping involves reducingthat density. It is believed that the dopant gas is relatively weaklyassociated with the organic semiconductor when the organic semiconductoris doped as described here, therefore making the doping reaction atleast partially reversible. The utility of this feature is discussed ingreater detail below.

When the organic semiconductor is heated in the presence of an ambientincluding a relatively high partial pressure of oxygen, e.g., theorganic semiconductor may be oxidized, and the concentration of oxygenand associated charge traps therein may increase. Conversely, when anorganic semiconductor already containing oxygen, e.g., is heated in anambient including a relatively low partial pressure of oxygen, theconcentration of oxygen in the organic semiconductor may be reduced.Thus, the concentration of charge traps associated with the oxygentherein may decrease, resulting in a reduced doping level.

In addition to the specific dopant gas and organic semiconductor used,the doping process may further depend on the pressure of the doping gasand the presence of other gases in the ambient. In an embodiment usingO₂ as the dopant gas, an oxygen partial pressure of about 50 kPa orgreater may be used for additive doping, and a partial pressure of about10 Pa or less may be used for subtractive doping. As used herein, anambient with a partial pressure of the doping gas about 10 Pa or less isreferred to as a vacuum. In another embodiment, when the doping gasincludes, e.g., oxygen, hydrogen in the ambient may act as a reducingagent, facilitating subtractive doping of a previously doped organicsemiconductor.

In an embodiment, the organic semiconductor includes rubrene. Rubrenemay be formed in crystalline form by, e.g., sublimation from a stream ofAr and/or H₂ gas. In a nonlimiting example, the rubrene is heated toabout 280-320° C. and the carrier gas is flowed through a horizontalsublimation tube. Rubrene crystals may form spontaneously in a 30 cmzone. A more detailed description of an example process is provided inU.S. patent application Ser. No. 11/159,781 to Kloc, et al., entitled“Purification of Organic Compositions by Sublimation,” which isincorporated by reference as if reproduced herein in its entirety. Asdescribed in detail below, FETs may be assembled using rubrene crystalsformed in this manner with advantageous results.

The inventors believe that the rate of change of semiconductingcharacteristics of the FET 100 may be limited by the rate of diffusionof the dopant gas into the organic semiconductor and the geometry of theFET 100 formed therewith. For example, for a fixed distance 170, ahigher diffusion rate of dopant gas in the channel material 110 willresult in a shorter time to achieve desired doping level. Similarly, fora fixed diffusion rate, a shorter distance 170 will result in a shortertime. As discussed further below, the duration of exposure to the dopantgas or vacuum may be limited to result in a desired conductivity of theorganic semiconductor.

As described briefly above, in some embodiments the dielectric layer 150is substantially impermeable to the dopant gas. In such embodiments, thedielectric layer 150, in combination with the substrate 195 when used,substantially prevents diffusion of the dopant gas into and out of thechannel material 110. The doping channel 165 is formed to provide a paththrough the otherwise impermeable barrier formed by the dielectric layer150 and substrate 195. The dopant gas may diffuse into or out of thedielectric layer 150 through the doping channel 165 and exposed portion160. When the channel material 110 is doped to a desired level, thedoping channel 165 can be sealed by the capping layer 197 or by apackage that excludes the dopant gas. The doping channel 165 exposes asmall fraction (e.g., about 10% or less) of the surface 115 of thechannel material 110, and the channel material 110 is consideredsubstantially encapsulated when this fraction remains small. In somecases, the channel material 110 is substantially encapsulated when atleast about 80% of the surface 115 of the channel material 110 iscovered with the dielectric layer 150. In other embodiments, about 90%or more of the surface 115 is covered. In yet other embodiments, 99% ormore of the surface 115 is covered.

The dielectric thickness 155 may be selected with two considerations inmind. First, the dielectric thickness 155 may be chosen to result in adesired V_(th). Second, the dielectric thickness 155 may be chosen suchthat the rate of diffusion of the dopant gas through the dielectriclayer 150 is low enough to provide stability of the doping level overthe lifetime of the FET 100. The minimum thickness necessary to resultin a particular diffusion rate will, in general, depend on the materialfrom which the layer is formed, and will typically be inversely relatedto the permeability of the dopant gas through a unit thickness of thematerial.

In some embodiments, the diffusion rate of the dopant gas in the channelmaterial 110 is low enough that the channel material 110 need not besubstantially encapsulated, or the barrier properties of the dielectriclayer 150 may be relaxed. In such cases, sealing the doping channel 165may also be optional. These embodiments are characterized by the dopantgas having a low enough diffusion rate in the channel material 110 atthe operating temperature that changes in doping over the life of theFET 100 may be neglected.

In one aspect, the dielectric layer 150 may be deposited in a mannerthat does not substantially alter the properties of the channel material110. In a nonlimiting example, the dielectric layer 150 is a polymer.Polymers may be deposited by, e.g., a spin-on or a chemical vapordeposition (CVD) process. One such polymer formed by CVD is Parylene N,in which oxygen, e.g., may have a permeability of about 1.3E-6μm²·s⁻¹·Pa⁻¹ to about 1.8E-6 μm₂·s⁻¹·Pa⁻¹ at about 23° C. Parylene N maybe deposited from the vapor phase in a highly conformal, pinhole-freeform. A thickness of 3-4 μm of Parylene N is often an effective oxygenbarrier at a temperature of about 100° C. or less. Substituted Parylenessuch as Parylene-C, D, or HT also have dopant gas permeability valuescomparable to or lower than Parylene N. Parylenes may be deposited atabout room temperature (25° C.), thus minimizing risk of heat-inducedchanges of the channel region 140.

FIG. 4 illustrates, without limitation, the relationship betweendrain-source current I_(ds) and gate-source voltage, V_(gs) of anexperimental FET configured as illustrated in FIG. 1A, using rubrene asthe channel material 110, parylene as the dielectric layer 150, and O₂as the dopant gas. The distance 170 from the doping channel 165 to thechannel region 140 for this device is about 1.6 mm, and the dielectricthickness 155 is about 3.2 μm. Data were obtained at saturation,V_(ds)=−40 V, to minimize the effect of contact potentials. Under theseconditions, I_(ds, sat)∝μ_(eff) (V_(gs)−V_(th))², where μ_(eff) is theeffective mobility of the holes. FIGS. 4A and 4B display I_(ds) ^(1/2)as a function of V_(gs). FIGS. 4C and 4D display log(I_(ds)) as afunction of V_(gs). FIGS. 4A and 4C are associated with oxidation of thegate dielectric, and FIGS. 4B and 4D are associated with reduction.Further details of this experimental work may be found in Woo-young So,et al., “Mobility-independent doping in crystalline rubrene field-effecttransistors,” Solid State Communications 483-86 (2007) incorporatedherein as if reproduced in its entirety.

Two additive doping (e.g., oxidation) steps are shown in FIG. 4A. Theinitial I_(ds) ^(1/2) versus V_(gs) characteristic is substantiallylinear below about −20 V, indicating that μ_(eff) is substantiallyconstant in this range. Doping was done with at a temperature of about110° C. and an O₂ ambient at about 110 kPa that substantially excludedother gases. After 10 h of oxidation, the transfer curve has shiftedsignificantly to the right, indicating that the FET V_(th) is lower, andthe conductivity of the channel region 140 is higher. Continuingexposure to O₂ and heat for an additional 17 h causes the curve to movefurther in the same direction, though less so. The 10 h and 27 h curvesare shifted without significant change of the slope, indicating that themajority carrier mobility is substantially unchanged after doping.

FIG. 4B shows the effect of subtractive doping (e.g., vacuum annealing)on transport behavior of the experimental FET. Annealing was done at atemperature of about 110° C. with an O₂ ambient at about 1.5 Pa. In thiscase, I_(ds) ^(1/2) versus V_(gs) was measured at 2, 10 and 18 hours ofannealing. In contrast to additive doping, in each case, the transfercharacteristic shifts left, indicating an increase of V_(th) and adecrease of conductivity of the FET. Again, the mobility of the chargecarriers is substantially unchanged, as inferred from the substantiallyunchanged slope during annealing. Moreover, after 18 hours of annealing,the FET is almost restored to the initial state, indicating substantialreversibility of the doping process.

In FIGS. 4C and 4D, the transfer curves are replotted on a logarithmicscale. The off-currents, e.g., I_(ds) taken at V_(gs)=0 V, are used toevaluate the equilibrium state of the channel material 110, in this caserubrene. After 27 h of oxidation, the off-current is seen to increase inFIG. 4C by about two orders of magnitude, while the current atV_(gs)=−60 V is seen to increase about 40% in FIG. 4A. Conversely,vacuum annealing for 18 h is seen to reduce the off-current by about oneorder of magnitude FIG. 4D, and is seen to lower the on-current by about25% in FIG. 4B. It is thought that in the on-state, the current of theFET 100 is governed not only by V_(gs) but also by the materialsparameters μ_(eff), n, and V_(th).

FIGS. 5A and 5B illustrate the mobility μ_(eff) (left axis), and V_(th)(right axis) of the experimental FET using rubrene as a function ofannealing time in O₂ (FIG. 5A) and vacuum (FIG. 5B). The conditions ofannealing are as described above in this nonlimiting example. The insetto each panel further illustrates the carrier concentration as afunction of annealing time. One sees in FIG. 5A that a higherconcentration of the dopant species, oxygen in this embodiment, reducesV_(th). This is believed to be attributable to inducing more holes inthe valence band. The μ_(eff) is also seen to remain substantiallyunchanged. FIG. 5B illustrates that a lower concentration of the dopantspecies partially restores V_(th) by removing some of the added holeswhile id, again remains substantially unchanged. It is believed that thesubstantially stable nature of μ_(eff) exhibited by the holes in thisexample illustrates that the transport mechanism of the channel material110 is substantially unaffected by the additive and subtractive dopingprocesses.

The conductivity a of the channel region 140 may be determined fromcurrent characteristics in FIG. 4 and knowledge of the geometry of thechannel region 140. The observed insensitivity of μ_(eff) with increaseda implies that the carrier density, n=σ/μ, increases with additivedoping, as shown in the inset to FIG. 5A. In this case, the data wereobtained from I_(ds) at V_(gs)=0 V. The doping effect is thus analogousto p-type doping in FETs of inorganic semiconductors where a similardecrease of threshold voltage with increased dopant density leavesμ_(eff) unchanged. This is consistent with the reported increase ofconductivity in rubrene on exposure to oxygen. See, e.g. V. Podzorov, etal., Appl. Phys. Lett. 85 (24)(2004) 6039, in which a light-mediateddoping reaction was used. The shift of V_(th) bears superficialsimilarity to that observed by V. Podzorov et al., Phys. Rev. Lett. 93(8) (2004) 086602, wherein V_(th) was found to increase upon exposure tox-rays. In the latter Podzorov work, however, the shift was associatedwith creation of deep-level traps, not with a known dopant species as inthe illustrated embodiment.

Turning to FIGS. 6A-6F, illustrated is a method of forming a FET 600. InFIG. 6A, an organic semiconductor 605 is shown with source/drainelectrodes 610 formed thereover. In another embodiment, not shown, theorganic semiconductor 605 is formed over the source/drain electrodes610. In a nonlimiting example, the organic semiconductor 605 is arubrene crystal formed by the method described previously. Thesource/drain electrodes 610 may be formed thereover by, e.g., conductivepaint or physical vapor deposition through a shadow mask. Theillustrated embodiment shows without limitation a free-standing crystalof the organic semiconductor 605 without a supporting substrate. Inother embodiments, not shown, the organic semiconductor 605 may beplaced on a substrate. The substrate may include other resistive orsemiconductor devices, and may be e.g., an inorganic semiconductorsubstrate or a flexible organic substrate.

FIG. 6B illustrates the FET 600 after formation of a dielectric layer615 is formed over the source/drain electrodes 610. The dielectric layer615 encapsulates the organic semiconductor 605 and the source/drainelectrodes 610. As described previously, the dielectric layer 615 maybe, in one aspect, substantially impermeable at the deposited thicknessto the dopant gas to be used. In some cases, the dielectric layer 615 isparylene or a substituted parylene. Connections to the source/drainelectrodes 610 may be made before or after the dielectric layer 615 isformed. If the connections are made afterward, openings (not shown) inthe dielectric layer 615 may be formed in a manner that does not exposethe organic semiconductor 605 to the ambient, such as, e.g., mask andplasma etch. In this embodiment, it is preferred that the source/drainelectrodes 610 are formed of a conductive layer impermeable to theintended dopant gas, such as a metal layer.

Also illustrated in FIG. 6B is a gate electrode 620 formed over thedielectric layer 615. As for the source/drain electrodes 610, the gateelectrode 620 may comprise a conductive paint or a metal layer. The gateelectrode 620 is positioned to produce a channel region (such as thechannel region 140, e.g.) that connects the source/drain electrodes 610when the FET 600 is operated. In some cases, a preferred configurationof the FET 600 has the gate electrode 620 at least coextensive with aspace 622 between the source/drain electrodes 610.

FIG. 6C illustrates the FET 600 after a doping channel 625 is formed inthe dielectric layer 615. In one embodiment, such as that illustrated inFIG. 1A, the organic semiconductor 605 may be cleaved and therebyexposed to the ambient. In other embodiments, such as those illustratedby FIGS. 1B and 1C, the doping channel 625 may be formed by, e.g., aplasma etch process to remove a portion of the dielectric layer 615. Thedoping channel 625 formed thereby may be located in any desired positionrelative to the source/drain electrodes 610. In some embodiments, thedoping channel 625 is located to minimize the required duration of anadditive or subtractive doping process in a later step. In otherembodiments, the doping channel 625 is formed before the gate electrode620 is formed.

In FIG. 6D, the FET 600 is subjected to a doping process 630. The dopingprocess 630 may include an additive doping process, a subtractive dopingprocess, or both. In some cases, the additive and subtractive dopingprocesses are an oxidation process and reduction process, respectively,as previously described. In some embodiments, the doping process 630 isperformed before the FET 600 is connected to other electricalcomponents. In other embodiments, discussed further below, the FET 600is connected to other components before performing the doping process630. In a nonlimiting example, the organic semiconductor 605 isadditively doped by heating to about 380 K (107° C.) in an O₂ ambient atabout 110 kPa. In another example, the organic semiconductor 605 issubtractively doped by heating to about 380 K in an ambient of about 10Pa of the dopant gas or less.

FIG. 6E illustrates the FET 600 after a first capping layer 635 isformed over the gate electrode 620. In the illustrated embodiment, thefirst capping layer 635 may be, e.g., a dielectric layer that issubstantially impermeable to the dopant gas. In this case, the firstcapping layer 635 serves to seal the doping channel 625 to substantiallyprevent the diffusion of the dopant gas into or out of the organicsemiconductor 605. In some embodiments, the first capping layer 635 isformed from the same material as the gate dielectric layer 615. In suchcases, an interface 636 may typically be detected by electronmicroscopy.

FIG. 6F illustrates the FET 600 after an optional second capping layer640 is formed over the first capping layer 635. The second capping layer640 may be substantially opaque to ultraviolet (UV) or longer wavelengthlight to prevent such light from illuminating the organic semiconductor605. This result may be desirable where illumination of the dopedorganic semiconductor 605 by such light causes dopant gas atoms tobecome disassociated with the organic semiconductor 605, thus changingthe conducting properties thereof. The light is substantially blockedwhen an insufficient fraction of the light reaches the organicsemiconductor 605 to cause a change of doping of the organicsemiconductor 605 over an operational lifetime. Substantially opaquemeans that the second capping layer 640 transmits less than about 5% ofincident light. In other cases, the transmission may be less that 1% or0.1%. In one example, the second capping layer 640 may be a metal orplastic portion of a package containing the FET 600, or may be an epoxyresin. In another example, the second capping layer 640 is a dielectricmirror, comprising multiple dielectric layers designed to result inreflection of a substantial portion of the light. In another example,functional aspects of the first capping layer 635 and the second cappinglayer 640 may be combined in a single material layer that blocks bothoxygen and light.

Turning to FIG. 7, illustrated is a flow diagram of a method 700 foradjusting a doping level of a FET such as the FET 600. In a step 710,the FET 600 is formed up to the point that the source, drain and gateelectrodes are formed, and a doping channel is formed in the gatedielectric layer. (See FIG. 6C, e.g.) In a step 720, electricalconnections to the FET 600 are made so that the FET 600 may be operatedin the manner anticipated by the intended application.

In a step 730 and a step 740, the doping level of the FET 600 isconfigured in a process referred to herein as “trimming”. The trimmingprocess is analogous to trimming of resistors in certain electronicsapplications. In step 730 the FET 600 is additively doped, whileoperating, as previously described to set a desired doping level of theFET 600. If the operating characteristic of the FET 600 is not within adesired range, the FET 600 may be further additively doped orsubtractively doped in step 740 to achieve the desired operatingcharacteristic. In some cases, trimming may include operating the FET600 in the manner anticipated by the intended application. In somecases, this may include operating the FET 600 in an electrical testsystem (a “test bed”) operated exclusively for the purpose ofdetermining the doping level. In a nonlimiting example, the test bed mayinclude an oscillating circuit with an operating frequency depending onthe doping level of the channel region 140. In other cases, theoperation may be in the actual circuit the FET 600 is to be operated inafter establishing the doping level. In such cases, the doping level maybe trimmed until the circuit operates within a design value range. Ofcourse, other methods of trimming the FET 600 may be used as appropriateto the application.

Trimming of the doping level provides an advantageous means of adjustingtransistor characteristics in a device or system where, e.g., it is notconvenient or feasible to fabricate the transistor with characteristicswithin a required operating range. For example, physical dimensionstypically are uncertain within a tolerance range. It is generally moreexpensive to produce devices with a tighter tolerance than those with alooser tolerance. The trimming process provides the means to manufacturewith a looser tolerance at lower cost, and then adjust the performanceof specific transistors whose performance causes the device or system tooperate outside a desired operating range. Also, regardless of thetolerance range, a manufacturing process may occasionally producestatistical outliers with performance outside the tolerance range. Thetrimming process provides for the adjustment of some outliers, allowingfor the recovery of devices or systems that might otherwise bediscarded.

Heating of the FET 600 may in some cases be done by any means compatiblewith the operation of the test system or the final circuit, such as,e.g., a heated chamber. In other cases, the heating may be done by asource of spatially confined energy. As used herein, energy is spatiallyconfined when it causes heating of all or a portion of the FET 600 whenprojected thereon, but does not significantly heat circuit componentsneighboring the FET 600, e.g., a distance about the same as thedimensions of the FET 600. In some cases, the heated area is smallerthan the FET 600. In other cases, the diameter of a region heated by thespatially confined energy source is about the length of the space 622 ofthe FET 600. Nonlimiting examples of sources of spatially confinedenergy include lasers and focused incoherent light. Lasers arecommercially available with a spot size on the order of one micron,providing the means to heat only a portion of the FET 600 when thedevice size is greater than 1 micron. When such a spot size is less thana feature size, e.g., the space 622, the spot may be scanned to heat theentire feature. Moreover, the wavelength of the light may be chosen inrelation to the size of features to be heated to help limit the spatialextent of heating. In some cases, the wavelength may also be limited toavoid producing molecular excitations in the organic semiconductor 605,as previously described.

The spot size, power density, wavelength and illumination time of thespatially confined energy source may be chosen to result in a desiredtemperature of a particular FET 600 without causing chemicaldecomposition of the organic semiconductor 605. In some embodiments, thebeam size and power are selected so that only one FET of a plurality ofFETs in a circuit is trimmed at a time. In other embodiments, multipleFETS are trimmed simultaneously. After the trimming operation iscompleted in steps 720, 730, the FET 600 may be sealed with one or morecapping layers, or packaged, to exclude a doping gas and/or light in astep 750.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

1. A method, comprising: forming a contiguous semiconducting region thatincludes polyaromatic molecules; heating said region to a temperatureabove room temperature in the presence of a dopant gas and the absenceof light to form a doped organic semiconducting region.
 2. The method ofclaim 1, further comprising forming a barrier to said dopant gas oversaid semiconducting region.
 3. The method of claim 1, wherein saiddopant gas includes oxygen.
 4. The method of claim 1, further comprisingforming a barrier that substantially prevents exposure of saidsemiconducting region to said dopant gas after said heating.
 5. Themethod of claim 4, wherein said barrier blocks light from illuminatingsaid region.
 6. The method of claim 4, wherein said barrier includes adielectric layer substantially impermeable to said dopant gas.
 7. Themethod of claim 1, further comprising forming a source and drainelectrode on a surface of said organic semiconducting region and a gateelectrode on a surface of said dielectric layer, thereby forming a FETtransistor.
 8. The method of claim 7, further comprising setting athreshold voltage of said FET transistor to a predetermined level afterforming said electrodes.
 9. The method of claim 8, wherein saidthreshold voltage is set by heating said polyaromatic molecules with asource of spatially confined energy.
 10. The method of claim 8, whereinsaid threshold voltage is set while operating said FET transistor. 11.The method of claim 1, wherein a mobility of majority charge carriers insaid semiconducting region changes by about 10% or less after dopingsaid semiconducting region.
 12. The method of claim 1, wherein moleculesof said region are selected from the group consisting of acenes andthiophenes.
 13. The method of claim 12, wherein said polyaromaticmolecules comprises rubrene.
 14. A method, comprising: forming anorganic semiconducting region that includes a crystalline region ofpolyaromatic molecules; forming a dielectric layer over said organicsemiconducting region; forming an opening in said dielectric layer toexpose said organic semiconducting region; and then heating saidcrystalline region to a temperature above room temperature in theabsence of light.
 15. The method of claim 14, wherein said heatingincludes exposing said crystalline region to a dopant gas comprisingoxygen during said heating.
 16. The method of claim 14, wherein saidheating includes exposing said crystalline region to an ambient with anoxygen partial pressure of about 10 Pa or less during said heating. 17.The method of claim 14, wherein said heating includes heating saidcrystalline region to a temperature ranging from about 90° C. to about150° C.
 18. A method, comprising: forming a crystalline organicsemiconducting region that includes polyaromatic molecules; placing asource electrode and a drain electrode in contact with said organicsemiconducting region; locating a gate electrode to affect theconductivity of said organic semiconducting region between said sourceand drain electrodes; forming a dielectric layer of a first dielectricbetween said organic semiconducting region and said gate electrode, saiddielectric layer being substantially impermeable to oxygen and incontact with said organic semiconducting region; and forming an openingin said first dielectric to form a doping channel wherein a portion ofsaid organic semiconducting region is in contact with a seconddielectric via said opening, a physical interface being located betweensaid second dielectric and the first dielectric.
 19. The method of claim18, further comprising heating said organic semiconducting region to atemperature above room temperature in the absence of light after formingsaid dielectric layer.
 20. The method of claim 18, wherein said seconddielectric is a solid dielectric material.